Only in my head of course. But I needed to get a whole bunch of round vases. And they needed to be a certain size. And the one I needed to match had a beautiful arrangement in it and I was not going to disturb it. But I couldn't tell what size the vase was. (It was very round, called a bubble bowl or bubble vase.)
I tried holding a tape measure in front to see how wide. Not easy, couldn't really tell. Then I tried a ruler in front and again behind. Numbers changed depending on my angle, eeek. Then I thought well I will just worry about the height, but again, lots of flowers in there that I was not going to touch. Hmm. I actually struggled with this for probably a day and a half. And then I had an 'Aha' geometry moment. (I think it is geometry.)
I remembered that there is some formula that says if you know one or two things you can get the other things. Mind you I did not know the formula by heart, but Google did. So I refreshed my mathness and saw that the diameter (which I needed) is equal to the circumference divided by PI. And the cool thing is that the circumfrence was 31.4-something inches. So very convenient! a 10" bowl. Yeah.
So when your kids say to you (as I said to anyone who would listen at the time) "WHY do I have to learn this stuff? You never use it in real life!" You can promptly correct them and relay my compelling saga. (I am so happy to be of service.)